Free flowing glass batch

ABSTRACT

A process for preparing and storing moist glass-making batch is disclosed, comprising incorporating a surfactant in moist batch so that when the batch is either: a) stored at a temperature below 35° C., it remains free flowing without setting, or b) pre-heated at, or above, 100° C. before supply to a glass melting furnace, it remains free flowing without setting. The moist batch includes from 2% to 10% by weight free water and from 0.0001% to 5% by weight surfactant, which is preferably a soluble soap (for example a carboxylate having from 4 to 22 carbon atoms in its chain) that is incorporated into the batch as it is mixed. The moist batch can be stored for at least 24 hours, and it can be pre-heated to at least 150° C., and remain free flowing without setting.

The present invention relates to a method of producing mixed, wet glassbatch that remains free flowing when subjected to a range oftemperatures, especially those below 35° C. and those above 100° C.

Throughout the glass industry there exists a problem when a decision hasto be made as to the type of batch that is to be fed into a furnace forproducing molten glass. The ultimate requirement in float glassmanufacture is a high quality end glass product, although issues such asminimisation of production costs, pollutant emissions and heat loss fromthe glass forming process must also be carefully considered. Factorsthat are likely to influence both the physical and chemical homogeneityand uniformity of the batch, and which therefore affect the quality ofthe final float glass product, include the degree to which batchmaterials are mixed as they enter the furnace and the physical conditionof the batch materials (for example, whether the batch is wet or dry).The most common defects that occur in float glass, which are on thewhole attributable to the consistency of the batch, are those of ream,bubble and inclusions.

Ream is a visible imperfection in the glass that manifests as streaksand changes in the refractive index. Problems with ream may be limitedto a small area of the glass or may extend over a large area of a floatglass sheet. Bubble is less of a problem than ream, but a problemnonetheless, and consists of blisters in the glass, which areeffectively gaseous inclusions that have been trapped in the moltenglass batch. The appearance of bubble is common when a batch mixturedoes not have the required homogeneity. An inclusion is the occurrenceof an unmelted batch particle in the final glass product. All three arehighly undesirable features that lead to glass being rejected due to itspoor quality.

A glass batch comprises a mixture of different raw materials, which areof various sizes, ranging from less than 60 μm to greater than 1 mm indiameter. The batch is usually wet to begin with (as the sand which isused in making the batch is usually wet), although in countries with acolder climate, the batch often has to be dried to prevent it freezingduring transportation or storage. With a dry glass batch it is clearthat this difference in batch particle size can lead to problems whenthe batch is transported from where it is mixed to a furnace:segregation of the batch is likely to occur and thus the pre-cursorconditions for occurrence of ream, bubble and inclusions in the finalglass product are likely to have been introduced. In addition, finerbatch particles often contribute to dusting problems in the batchsystems and carry-over problems in the furnace.

The problem of batch segregation may be minimised if the original batchremains wet (rather than being subjected to a drying process).Maintaining the batch in a wet condition minimises dusting and reducesfurnace carry-over problems because the batch particles are boundtogether. Furthermore, a wet glass batch exhibits improved meltingcharacteristics of the batch ingredients, compared to a dry batch,thereby assisting to eliminate the occurrence of the type of glassimperfections that were discussed earlier. The use of wet batch toreduce dusting and enhance melting is described in U.S. Pat. No.3,294,555 to S M Krinov. Krinov teaches the use of water additions inthe range of 1% to 3% to reduce dusting and improve the uniformity ofthe glass batch, and use of higher proportions of water, in particular5% to 20%, to additionally enhance the melting characteristics of thebatch, increasing the melting rate. The increased melting rate is saidto contribute to reduction of blister (bubble) in the melted glass.

The advantages of wetting glass batch are, unfortunately, accompanied byserious disadvantages. Thus, as explained by Krinov, the use of water(especially in the high proportions taught to enhance melting) rendersthe batch susceptible to hardening or setting such that pneumatichammers ‘and other types of traumatic equipment’ are required to breakup the batch for use. Krinov teaches overcoming the tendency of themoist batch to harden or set by mixing the moist batch cold; the solidcomponents of the batch are cooled before mixing, preferably to atemperature in the range 10° F. to 30° F., the water used to wet thebatch is cooled by refrigeration, preferably to 35° F. to 40° F., andthe wet batch is cooled during mixing to maintain the temperature below70° F. despite the exothermic reactions which occur on the addition ofwater. Krinov suggests that, by carrying out the mixing at lowtemperature, the reaction of the water with soda ash present in thebatch to form higher hydrates (the heptahydrate and decahydrate) takesplace during the mixing operation. If formed after mixing, the hydrateswould tend to bind the sand and other glass components setting thebatch. Cooling during mixing apparently results in the hydrate crystalsbeing formed, and broken up, during the mixing operation, and there isno tendency for the broken crystals to re-orient and set the batch aftermixing.

The Krinov process requires special cooling procedures and is believednot to have been widely used. An alternative, and more widely used,approach to providing high free moisture batch while avoiding setting orhardening of the batch is described by Lehman and Manring in ‘GlassBatch Wetting with Water’ (The Glass Industry, December 1977, pages16-34). According to Lehman and Manring, the temperature of the wetbatch is maintained above 35.4° C. (the dehydration point of sodiumcarbonate heptahydrate) to avoid the formation of both the heptahydrateand the decahydrate and minimise the loss of water taken up as water ofhydration. However, this technique, which is now in common use, requiresmoist batch to be stored at a temperature of over 35° C., preferablyover 40° C., in order to avoid formation of a higher hydrate and settingof the batch. Moreover, a serious disadvantage of this moist batch isthat, if attempts are made to preheat it before supply to the furnaceusing hot waste gases from the furnace (with consequent saving ofenergy) it is found to set solid.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of producing moistglass batch which can be stored below 40° C., especially below 35° C.,without unacceptable hardening or setting and without the need forrefrigeration of the water or cooling of the solid batch ingredientsbefore or during mixing.

It is a further object of the invention to provide a moist free flowingglass batch that can be preheated before being fed to a glass-meltingfurnace without unacceptable hardening or setting of the batch.

The present inventor has found that both these requirements may be metby incorporating a surfactant in the mix, conveniently by adding it towater used to moisten the batch. Although the prior art contemplates theinclusion of a wetting agent or surfactant in the moist batch to enhancewetting of the batch, there is no suggestion in the art that use of asurfactant would enable either of the problems discussed to be overcome.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, the present invention will now be moreparticularly described by reference both to the following non-limitingexamples and to the accompanying FIGS. wherein:

FIG. 1 is a graph of the weight (in grammes) required to penetrate abatch cone (the ordinate) against the percentage by weight of soap insolution based on the weight of the soap solution (the abscissa) forExample 2;

FIG. 2 is a graph of the weight (in grammes) required to penetrate abatch slab (the ordinate) against the percentage by weight of soap insolution based on the weight of the soap solution (the abscissa) forExample 3, and

FIG. 3 is a graph of the weight (in grammes) required to penetrate abatch slab (the ordinate) against the percentage by weight of soap insolution based on the weight of the soap solution (the abscissa) forExample 4.

DETAILED DESCRIPTION OF THE INVENTION

According to a first aspect of the invention, there is provided aprocess for preparing and storing moist glass-making batch whichcomprises incorporating a surfactant in moist batch whereby, on storageof the batch at a temperature below 35° C., the batch remains freeflowing without setting.

According to a second aspect of the invention, there is provided aprocess for preparing moist glass-making batch which comprisesincorporating a surfactant in the moist batch whereby, on preheating thebatch at or above 100° C. before supply to a glass melting furnace, thebatch remains free flowing without setting.

The expression ‘moist’ batch is used to denote batch containing 1% to20% by batch weight of free water; ‘free water’ is water which is drivenoff when the batch is heated under atmospheric pressure at a temperatureof at least 110° C., but no greater than 600° C. (at which thermaldecomposition of other components of the batch may occur). It ispreferred to use batches containing at least 2% free water in order tooptimise the melting process, while it is generally preferred to avoidthe use of more than 10% free water because, the more water is present,the more heat is required to drive off the water during melting.Furthermore, above 10% free water, batch handling becomes difficult andthe water begins to drain from the batch. For optimising meltingperformance consistent with economic use of heat, the glass batch usedwill normally contain at least 3%, and no more than 7%, free water.

The surfactant to be incorporated into the moist batch may be chosenfrom any one of the following four classes of surfactant: anionic,cationic, non-ionic and zwitterionic. Preferably the surfactant isanionic; examples from this class of surfactant tend to have the mostpronounced effect upon inhibition of batch hardening, and are thus mosteffective at achieving a moist batch that remains free flowing withoutsetting. It is further preferable that the anionic surfactants used aresoluble soaps. The soluble soaps used are preferably carboxylates havinga carbon chain length of between C4 and C22 inclusive, and furtherpreferably having a Group I, II or III counter-ion, especially an alkalimetal counter-ion.

The surfactant is preferably incorporated into the batch in liquid form;the amount of liquid typically being 4% of the batch weight. The amountof surfactant added to the batch, as a percentage of the batch weight,is preferably at least 0.0001% so that a discernible effect may beobserved as compared to when water alone is used to wet the batch, butno greater than 5% because above this value, the advantage conferred bythe surfactant does not increase significantly. Furthermore, at apercentage by batch weight of greater than 5% any cost advantage ofusing a surfactant in the processes of the present invention begins todisappear when compared to the cost of, for example, maintaining thetemperature of the batch above 35° C. When the percentage by batchweight of the surfactant added to the batch is at least 0.01% and nogreater than 1%, especially 0.02% to 0.5%, a significant degree ofinhibition of the hardening process is observed over a range ofsurfactants from the different classes. It is most preferable, however,that the surfactant be added at 0.05% of the batch weight because thebenefit of its use is maximised with respect to the cost of using it inthe processes of the present invention.

The batch materials of the present invention may be mixed and melted inappropriate ratios in generally conventional manner to provide therequired glass composition. In the case of float glass, one example of atypical composition (percentages by weight) would be: 72% SiO2, 1%Al₂O3, 0.1% Fe2O3, 13.5% Na2O, 0.6% K2O, 8.5% CaO, 4% MgO and 0.2% SO3.The batch ingredients may be mixed at the ambient temperature of theirsurrounding environment with a surfactant and water withoutrefrigeration or even cooling before or during mixing. Indeed, steam maybe used to moisten the batch ingredients instead of cooler water, inwhich case the temperature at which the batch ingredients is mixed willbe in the region of between 50° C. and 100° C., depending on factorssuch as the initial temperature of the batch.

Mixing the batch with steam instead of water is advantageous if thebatch is to be further pre-heated. Once mixed, the batch may either befed directly into a glass making furnace, it may be transported to aglass-making furnace if mixing took place elsewhere, or it may be putinto storage until it is needed. Mixed batch that has been wetted withsurfactant and water according to the present invention can be stored attemperatures of below 35° C. or less, for example below 30° C. or evenbelow 25° C., for days, weeks or even months without any significantdegree of hardening being observed.

As has been mentioned earlier, the surprising effect that a surfactanthas when incorporated into a glass batch is additionally important whenapplied to batch pre-heating systems, wherein furnaces recycle theexhaust gases that result from fuel combustion within the furnace topre-heat the batch materials before they enter the furnace. Exhaustgases may have temperatures in the region of 1400° C. upon immediateexit of the furnace; thus recycling this heat is a cost-effectivemeasure. Batch is typically pre-heated to temperatures of at least 150°C. (preferably 300° C.) over a period of at least 12 hours (preferably24 hours). A batch pre-heater such as is claimed by The BOC Group plc inEuropean Patent application number EP 1123903 A2 would be the currentpre-heater of choice because it additionally utilises electrostaticforces to remove fine particulate matter from the exhaust gases, thusachieving pollution emission reduction. Prior to the present inventionhowever, it was not possible for moist batch to be successfullyintroduced into such a pre-heater system because such wet batch that hasbeen wetted without a surfactant exhibits hardening when it is exposedto higher temperatures, especially 100° C. and above.

A wet glass batch that results from the present invention is widelyapplicable throughout all areas of the glass making industry. A wetglass batch that remains free flowing without setting when subjected totemperatures either below 35° C. or above 100° C. has utility in theflat glass industry, especially in producing float and rolled glasses,in the container glass industry, especially bottles but also tubes(including cathode ray tubes), and also in glass fibre production.

For a better understanding, the present invention will now be moreparticularly described by reference to the following non-limitingexamples.

EXAMPLE 1

The first experiment to assess the performance of a soap solution foruse in a method of the invention was based on the following test. A 150g sample of dry glass batch was made up as follows: sand—91.9 g; sodaash—27.8 g; dolomite—22.8 g; limestone—6.4 g; saltcake—1.09 g. Thissample was then thoroughly mixed for 20 minutes at typical roomtemperature (around 20° C.). 6 ml of water was added to the thoroughlymixed dry batch, and the resultant wet batch was then again thoroughlymixed whilst remaining at typical room temperature (about 20° C.). Thewet batch was moulded into a cone having a height of 8 cm and a basediameter of 5 cm. The wet batch cone (having a temperature of less than35° C.) was then left to stand for one hour at typical room temperature.After this period had elapsed, an indentor probe (of 4 mm diameter) wasplaced centrally on top of the batch cone, successive weights were addedand the maximum weight that the batch cone could support was determined.The procedure was repeated using, in place of the water, dilute aqueoussolutions of soap derived from palm kernel acid. When only water isused, the batch cone can sustain over 250 g on the probe. When soapsolution is used, the batch cones lose their integrity after 5 minutesmaking the indentor probe test impossible to perform, but clearlyshowing the effectiveness of the surfactant in rendering the batch freeflowing.

EXAMPLE 2

The performance of a soap solution in maintaining the moist batch freeflowing when it is pre-heated prior to supply to a glass melting furnacewas experimentally assessed by testing samples of a number of the soapspecies contained therein as the surfactant of choice. To determine theeffectiveness of solutions of these soap species upon the degree ofhardening exhibited by an initially wet batch sample after heating, aprocedure as described above was followed to produce moulded cones ofmoist batch, which were transferred to a hot plate at 300° C. and leftfor 30 minutes. When the 30 minutes had elapsed, the batch cones weresubjected to the same indentor probe test as above. In this way, theperformance and effect of each soap species upon inhibition of batchhardening could be observed.

To make the various soap solutions for each of the chosen soap species,an initial “mother soap solution” was prepared. The mother solution wasdiluted until its effect on the cone strength was indistinguishable frompure water. Each volume of diluted soap solution in turn was used as thesolution from which the 6 ml of liquid in the tests described above wastaken. For each species of soap solution the maximum weight that thebatch cone could support after being wetted and subsequently heated asdescribed in the test above was recorded; the results are shown in FIG.1, in which the ordinate represents the weight (in grammes) required topenetrate a batch cone, the abscissa represents the percentage by weightof soap in solution based on the weight of the soap solution, and thesoap species are: (a) palm kernel acid potassium soap, (b) potassiumoleate, (c) potassium stearate, (d) potassium caprylate, (e) potassiumlaurate.

All the soaps tested were readily soluble in water, with the exceptionof potassium stearate. Each of the potassium soaps tested can be seen toreduce the batch cone strength. When only water is used, the batch conecan sustain over 1 kg on the probe. This is reduced to around 5 g whenthe soap solutions are used.

EXAMPLE 3

The test procedure in this Example was similar to that described inExample 2, except that, to promote heat transfer from the hot plate tothe batch sample, the batch sample was formed into a circular slab, 2 cmhigh and 6 cm diameter. The strength of the batch slabs was once againmeasured by placing an indentor probe (of 4 mm diameter) centrally ontop of the batch slab and then adding successive weights. The batch usedto form the slabs in this experiment was made up as follows: sand—91.9g; soda ash—28.6 g; dolomite—22.8 g; limestone—5.6 g; gypsum—1.04 g. Anumber of additional soap species to the soap species used in Example 2were synthesised (i.e. having a carbon content from C4 to C22) fortesting. Again, the performance and effect of each soap upon inhibitionof batch hardening could be observed. The results of the tests on thestrength of batch slabs as a function of the strength of soap solutionare shown in FIG. 2, in which the ordinate represents the weight (ingrammes) required to penetrate a batch slab, the abscissa represents thepercentage by weight of soap in solution based on the weight of the soapsolution, and the soap species are: (a) soap derived from the potassiumsalt of trans-2-decanoic acid, (b) potassium laurate, (c) soap derivedfrom the potassium salt of octanoic acid, (d) potassium oleate, (e) soapderived from the potassium salt of capric acid, (f) potassiumricolinoleate. Changing the carboxylate counter ion to sodium was alsofound to have the desired effect of preventing the batch from hardening.Both sodium and potassium soaps are examples of anionic surfactants.Other anionic surfactants were also tested, including sodium dodecylsulphate and sodium docylbenzensulfonate, and showed beneficial effectsin rendering the batch free flowing on storage and/or pre-heating.

EXAMPLE 4

Samples of three other classes of surfactant (cationic, non-ionic andzwitterionic) were further tested as the surfactant of choice.Dodecyltrimethylammonium chloride is a typical cationic surfactant andwas assessed using both strength tests described above. There is a widerange of non-ionic surfactants based on ethylene oxide-propylene oxideblock copolymers. A sample of Synperionic PE-68 was tested for itseffectiveness on inhibition of the batch hardening process. Zwitterionicdetergents are relatively rare,3-(dodecyidimethylammonio)propanesulfonate inner salt was obtained andalso assessed in the strength tests. FIG. 3 presents the results of thetests completed with these examples of other classes of surfactants; thebatch slabs were made and tested according to the procedure given inExample 3. In FIG. 3, the ordinate represents the weight (in grammes)required to penetrate a batch slab, the abscissa represents thepercentage by weight of soap in solution based on the weight of the soapsolution, and the soap species are: (a) soap derived from the potassiumsalt of trans-2-decanoic acid, (b) sodium dodecylbenzene sulphonate, (c)sodium dodecyl sulphate, (d) dodecyltrimethylammonium chloride, (e)potassium oleate, (f) Synperionic PE F-68, (g) sodium oleate, (h)potassium butyrate, (i) dodecyl dimethylammonio-sulfonate, (j) sodiumlignosulphonate.

In general, while the anionic surfactants perform better than the otherclasses of surfactant, a significant reduction in batch strength isfound when examples of the other classes of surfactant are used.

EXAMPLE 5

The performance of a soap solution derived from palm kernel acid inmaintaining moist batch free flowing when it is pre-heated, prior tosupply to a glass melting furnace, was assessed in a full-scale planttrial. A batch pre-heater substantially as described and illustrated(especially in FIGS. 1 and 6) in European patent application number EP1123903 was used. Dry glass batch including (in an approximate ratio) 87parts sand, 26 parts soda ash, 22 parts dolomite, 6 parts limestone and1 part saltcake, was fed into a batch hopper. As the batch moved along achannel towards a batch pre-heater, it was sprayed with a 1.75%concentration soap solution whilst being continually mixed so as toachieve uniform distribution of the soap solution. Upon introduction tothe batch pre-heater, the wet batch had a total moisture content of 4%and was at a temperature of around 36° C. The batch was then subjectedto heating from a furnace waste-gas stream directed into the bottom ofthe batch pre-heater, which was at a temperature of around 570° C. Asthe wet batch moved downward through the pre-heater, its temperatureincreased thus driving off the water and drying the batch. The dry batchwas at a temperature of over 300° C. as it exited the pre-heater, andmost importantly it was free-flowing and had remained so as it passedthrough the pre-heater.

1. A process for preparing and storing moist glass-making batch whichcomprises incorporating a surfactant in moist batch and storing thebatch at a temperature below 30° C. whereby, on storage of the batch ata temperature below 30° C., the batch remains free flowing withoutsetting.
 2. A process as claimed in claim 1 wherein the moist batchincludes between 2% and 10% free water.
 3. A process as claimed in claim1 wherein the moist batch includes a water-soluble component.
 4. Aprocess as claimed in claim 3 wherein the water-soluble component issoda ash.
 5. A process as claimed in claim 1 wherein the surfactant ischosen from a group consisting of anionic, cationic, non-ionic andzwitterionic surfactants.
 6. A process as claimed in claim 5 wherein thesurfactant is anionic.
 7. A process as claimed in claim 6 wherein theanionic surfactant is a soluble soap.
 8. A process as claimed in claim 7wherein the soluble soap is a carboxylate having a carbon chain lengthof between C4 and C22 inclusive.
 9. A process as claimed in claim 8wherein the soluble soap includes a Group I, II or III counter-ion. 10.A process as claimed in claim 1 wherein the surfactant is incorporatedinto the batch in an amount from 0.0001% to 5% of the weight of thebatch.
 11. A process as claimed in claim 1 wherein the surfactant isincorporated into the batch as the batch is mixed.
 12. A process asclaimed in claim 1 wherein the moist batch is stored for at least 24hours.
 13. A process for preparing moist glass-making batch whichcomprises incorporating a surfactant in the moist batch and preheatingthe batch at or above 100° C. whereby, on preheating the batch at orabove 100° C. before supply to a glass melting furnace, the batchremains free flowing without setting.
 14. A process as claimed in claim13 wherein the batch is pre-heated to a temperature of at least 150° C.before supply to the glass melting furnace.
 15. A process as claimed inclaim 13 wherein the moist batch includes between 2% and 10% free water.16. A process as claimed in claim 13 wherein the moist batch includes awater-soluble component.
 17. A process as claimed in claim 16 whereinthe water-soluble component is soda ash.
 18. A process as claimed inclaim 13 wherein the surfactant is chosen from a group consisting ofanionic, cationic, non-ionic and zwitterionic surfactants.
 19. A processas claimed in claim 18 wherein the surfactant is anionic.
 20. A processas claimed in claim 19 wherein the anionic surfactant is a soluble soap.21. A process as claimed in claim 20 wherein the soluble soap is acarboxylate having a carbon chain length of between C4 and C22inclusive.
 22. A process as claimed in claim 20 wherein the soluble soapincludes a Group I, II or III counter-ion.
 23. A process as claimed inclaim 13 wherein the surfactant is incorporated into the batch in anamount from 0.0001% to 5% of the weight of the batch.
 24. A process asclaimed in claim 13 wherein the surfactant is incorporated into thebatch as the batch is mixed.
 25. A process as claimed in claim 13wherein the moist batch is stored for at least 24 hours.